1. Field of the Invention
This invention relates to an apparatus and an method for finely dividing a molten metal by atomization.
2. Description of the Prior Art
In the production of grinding diamond wheels, tipped tools of high-speed steel, preforms of machine parts for hot isostatic pressing, preforms for injection molding, etc. by powder metallurgy, metal powders used as starting materials are required to have an average particle diameter of several micrometers.
Previously, the oxide reduction method, the electrolytic method, the carbonyl method, etc. have been known for the production of metal powders. These methods are suitable for the production of powders of a single metal, but for the production of fine powders of alloys, have the defect that restrictions on the compositions of the alloys make it difficult to powderize them, and the cost of production becomes high.
The atomizing method has been widely used for the production of alloy powders. The average particle diameter of the alloy powders produced by this method is several tens of micrometers at the smallest, and it has been considered impossible to produce alloy powders which are 1/10 times smaller.
For the powderization of a molten metal by the atomizing method, a conical jet process is considered most effective which comprises using a liquid, generally water, as a atomizing medium and concentrating the energy of a jet of the atomizing medium on one point. An apparatus of the type shown in FIG. 1 is known to be used in this process (see the specification of Japanese Patent Publication No. 6389/68). With this apparatus, a water jet is propelled from an annular zone defined by an outside nozzle jacket 7 and an inside nozzle jacket 8 to form a conical surface having a convergence point at one point 0 on the axis of the annular zone. The jetting of the liquid is caused by pressure from a liquid introducing pipe 9. In the meantime, a molten metal is let fall as a molten metal stream 13 from a molten metal feed nozzle 12. The air pressure is usually a negative pressure of 10 to 100 torr inwardly of the conical surface formed by the water jet, namely in the vicinity of the jet on the molten metal flowing side, and the molten metal stream 13 is sucked toward that site without swaying. On the other hand, the air pressure exteriorly of the jet is nearly 1 atmosphere. The average particle diameter of the resulting metal powder becomes smaller as the jetting pressure (speed) of the jet and the apex angle .theta. of the cone become larger. Industrially, the atomizing is carried out at a maximum jetting pressure of 200 kgf/cm.sup.2 and a cone apex angle of 20.degree.&lt;.theta..ltoreq.40.degree.. If the apex angle .theta. is further increased, the jet stream flows backward from the convergence point 0 to blow the molten metal upwardly, and the atomizing can no longer be continued. The critical apex angle, which is the largest .theta. value at which the atomizing can be continued, becomes smaller as the jetting pressure becomes higher.
Japanese Laid-Open Patent Publication No. 114467/1979 discloses a similar apparatus in which the .theta. value is increased. In this apparatus, a long suction pipe adhering intimately to the bottom of the nozzle is provided concentrically with the axis of the jet. By propelling the jet into this pipe, the amount of air flow sucked together with the molten metal from the upper portion of the nozzle is increased and the backward flowing of the jet from the convergence point is suppressed. Consequently, the atomizing can be effected while maintaining the apex angle at 80.degree.&lt;.theta.&lt;120.degree.. With this apparatus, however, the jet gets mixed with the air sucked in a large amount by the action of the suction pipe. As a result, it expands to the diameter of the suction pipe and decreases in density. The jet energy acting on powderization is inevitably decreased. When the jetting of water is carried out in an inert gas, a large amount of the inert gas is consumed, and therefore, the apparatus is not well adapted for water jetting in an inert gas atmosphere.